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The Cook Inlet beluga acoustics (CIBA) research program was initiated in 2008 with the primary objective of examining the year-round distribution of belugas and other odontocetes in Cook Inlet, Alaska, based on passive acoustic monitoring. This research program included substantial efforts to design and test passive acoustic mooring packages that would maximize the chances of successful long-term data collection in the particularly challenging Cook Inlet environment.

Currently, information on year-round distribution of belugas in Cook Inlet is very limited, in part due to the logistical challenges of working in extreme environmental conditions in Cook Inlet. Specifically, Cook Inlet is a subarctic estuary dominated by one of the largest tidal ranges observed in coastal regions of the world’s oceans. Tidal ranges in the upper inlet, the core beluga habitat, can exceed 10 m, which produces strong currents and tidal bores with speeds up to 5 m/s. As much as 500 km2 of mud flats, which are extensively used by belugas at high tide, are flooded and exposed twice daily by the large tides. The water has very high turbidity; thus, belugas are only visible when at the surface. Most of the coastline and the shallow mudflats in the upper inlet are inaccessible to boats when flooded and dangerous to traverse when exposed due to the unstable silt composition and the violent flooding currents. The upper inlet receives major drainage from southcentral Alaska, resulting in the deposit of massive amounts of glacial sediment which is subsequently transported from one place to another on a daily basis by the strong tidal currents. Therefore, acoustic moorings are subjected not only to extreme hydrodynamics, but also to a considerable risk of burial. Additionally, mooring exposure to suspended sediment, from fine silt to gravel, during strong current periods increases the risk of instrument damage or leakage and generates high levels of self-noise in the acoustic recordings, caused by the continuous collisions of suspended substrate with the mooring structure and instrument housings. Furthermore, vegetative debris from glacial river outflow, particularly during flooding tides, increases the risk of mooring entanglement and instrument damage. To complete this suite of challenges, from November to late April, the surface of the upper and mid inlet is covered in ice, with pressure ridges as thick as 6 m, and is in constant mechanical stress due to tide and current conditions. Dynamic, shifting ice increases the risk of ice scouring in the majority of the upper inlet because of the dominant shallow water conditions, which can result in moorings being ripped away from their anchors. All of these factors presented substantial challenges in designing acoustic moorings that would survive approximate 6-month deployments in Cook Inlet.

To increase our chances of acoustically detecting belugas in this naturally noisy environment, two instruments were used in tandem in our moorings, each one focusing on a different frequency range: one for social signals (1-12 kHz) and the other for echolocation (20-160 kHz). To our knowledge, this is the first time that this approach has been applied.

Figure 1. a) Streamlined, subsurface, mooring (Open Seas Instrumentation SUBS) configuration initially used by the CIBA research program in 2008 to deploy passive acoustic instruments and study beluga year-round presence in Cook Inlet, Alaska; b) Standard, linear-mooring configuration used in 2009. These mooring configurations were not successful as the moorings were lost; c) barely resurfaced entangled in vegetation; or d) were recovered with damaged instruments.

The project started in upper Cook Inlet in 2008, testing a streamlined, subsurface, mooring (Open Seas Instrumentation SUBS) configuration typically used in tidal current surveys (Fig. 1a); but this approach was unsuccessful, most likely due to entanglements with vegetative debris. This mooring type was replaced in 2009 with a standard, linear-mooring configuration, with the acoustic sensors attached one above the other to a mooring line with an acoustical release, a sacrificial anchor, and a subsurface float (Fig. 1b). However, in the upper inlet, several moorings of this design were lost, barely resurfaced entangled in vegetation (Fig. 1c), or had damaged instruments (Fig. 1d), resulting in minimal data collection. In early 2010, the linear-mooring configuration was replaced by a more compact, lower profile, “sandwich” configuration, with the acoustic sensors and acoustical release arranged side by side in a lower profile cluster (Fig. 2a).
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CIBA is composed of members from the Alaska Department of Fish and Game, Alaska Fisheries Science Center’s National Marine Mammal Laboratory, Hawaii Institute of Marine Biology, University of Alaska Fairbanks, Department of Defense Joint Base Elmendorf Richardson, and Prince William Sound Science Center. Delmar Westerholt, Terrasond Limited, was instrumental in mooring design and construction for CIBA.